1. Field of the Invention
The present invention relates to a method for fabricating a Complementary Metal Oxide Semiconductor (xe2x80x9cCMOSxe2x80x9d) Thin Film Transistor (xe2x80x9cTFTxe2x80x9d). More particularly, the present invention relates to fabricating a CMOS TFT with a simplified method which eliminates the ion implantation and annealing steps used in present methods.
2. State of the Art
The development of new portable electronic products, such as notebook computers and personal interface devices, is currently receiving a great deal of attention in the consumer products market. Substantial research and development have been focused in the field of active matrix liquid-crystal displays. Active matrix displays generally consist of flat panels of liquid crystals or electroluminescent materials which are switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d by electric fields. Every liquid crystal picture element (xe2x80x9cpixelxe2x80x9d) is charged by thin film transistors (xe2x80x9cTFTsxe2x80x9d). One of the most common components in the circuit for the liquid crystal display is the inverter, which has a CMOS structure constructed with a pair of n-type and p-type TFTs.
Although both polysilicon and amorphous silicon may be used in the fabrication of TFTs, a polysilicon semiconductor layer generally provides greater mobility of electrons and holes than does amorphous silicon. Furthermore, the CMOS structure can be easier to construct with polysilicon since the n-type and p-type TFTs can be formed by the same implant and anneal process. The CMOS inverter constructed with polysilicon TFTs also offers excellent characteristics in terms of operating frequency and power consumption.
In the fabrication of a TFT, the semiconductor-layer source and drain regions are formed by introducing an impurity element into the semiconductor layer (see U.S. Pat. No. 5,514,879 issued May 7, 1996 to Yamazaki). The controlled introduction of impurities enables good transistor characteristics. Typically, the introduction of impurities for a CMOS TFT requires two masking and implantation steps. As shown in FIG. 26, a substrate 202 is coated with a layer of oxide 204. A non-doped polysilicon layer 206 is applied to the oxide layer 204, and a resist layer 208 is applied to the non-doped polysilicon layer 206 in a predetermined pattern. The non-doped polysilicon layer 206 is then etched to form a non-doped polysilicon ledge 210, as shown in FIG. 27. A portion of the non-doped polysilicon ledge 210 is masked with a first mask 212 and an n-type impurity is introduced into the unmasked portion of the non-doped polysilicon ledge 210 to form an n-type area 214, as shown in FIG. 28. The first mask 212 is removed and a second mask 216 is applied to the non-doped polysilicon ledge 210 so as to cover the n-type area 214 and a portion 220 of the non-doped polysilicon ledge 210. A p-type impurity is introduced to the unmasked portion of the non-doped polysilicon ledge 210 to form a p-type area 218, as shown in FIG. 29. The second mask 216 then is removed to form a fundamental CMOS TFT gate structure 222 with the portion 220 acting as an insulating barrier between the n-type area 214 and the p-type area 218, as shown in FIG. 30.
The impurities can be introduced by thermal diffusion or ion implantation. By using thermal diffusion, the impurities are introduced from the surface of the semiconductor layer. By using ion implantation, impurity ions are implanted into the semiconductor layer. The ion implantation method provides a more precise control with respect to the total impurity concentration and depth that the impurities can be implanted into the semiconductor layer, and thus allows impurities to be implanted into a shallow, thin film. Furthermore, ion implantation can be performed at low temperatures. For the above reasons, ion implantation is a preferred technique for introducing impurities into a semiconductor layer in the fabrication process of the TFT.
In the above-described fabrication process of the TFT, impurities are implanted by a conventional ion implantation apparatus using an ion beam having a diameter of only several millimeters. When the ions are to be implanted over a large substrate using the above conventional ion implantation apparatus, it is necessary to either move the substrate mechanically or scan the ion beam electrically over the substrate since the area of the substrate is larger than the diameter of the ion beam. The necessity of having a mechanical moving means for the ion beam causes a problem in that the ion implantation apparatus becomes complicated, large, and expensive.
One technique for solving the above problem, wherein ions can be easily implanted into a large area, is an ion shower-doping method. According to this technique, ions generated by using a plasma discharge are dispersed in a cone shape and accelerated at a low voltage without mass separation to implant in the substrate. Although this technique allows ions to be implanted simultaneously over a large portion of the entire semiconductor layer, it does not result in uniform implantation.
Once the implantation is complete, the structure is annealed at about 600xc2x0 C. to activate the impurities. However, the temperature of annealing is detrimental to any temperature-sensitive portion of the entire structure.
Therefore, it would be advantageous to develop a technique to form a CMOS TFT which eliminates the need for introducing impurities during the fabrication of the CMOS TFT, while using state-of-the-art semiconductor device fabrication techniques employing known equipment, process steps, and materials.
The present invention relates to a method for fabricating a CMOS TFT using doped and activated n-type and p-type polysilicon layers, which eliminates ion implantation and annealing steps used in present methods.
In one embodiment of the present invention, the CMOS TFT is constructed by first coating a base substrate with a layer of oxide. A pre-doped and pre-activated n-type polysilicon layer is applied over the oxide layer. A resist layer is applied to the n-type polysilicon layer such that when the n-type polysilicon layer is etched and the resist layer is removed, n-type regions are formed. An isolation material layer is then applied over the oxide layer and the n-type regions. The isolation material layer is etched such that a portion of the isolation material layer, which resides in the corners formed at the junction between the oxide layer and the n-type regions, forms isolation caps. A pre-doped and pre-activated p-type polysilicon layer is then applied over the oxide layer, n-type region, and isolation caps.
The p-type polysilicon layer is then planarized down to form the p-type regions having a height substantially equal to that of the n-type regions. The p-type regions and the n-type regions are isolated from one another by the isolation caps, hereafter referred to as the isolation barriers. It is, of course, understood that the p-type layer may be applied to the oxide layer prior to applying the n-type layer.
The n-type regions and p-type regions may alternately be formed by again coating a substrate with a layer of oxide. A pre-doped and pre-activated n-type polysilicon layer is applied to the oxide layer. A pad oxide is applied over the n-type polysilicon layer and a nitride layer is applied over the pad oxide to form a layered assembly. A resist layer is patterned on the nitride layer. The layered assembly is etched and the resist layer removed to form an n-type region stack. The nitride layer is then removed and a thin layer of silicon dioxide is formed over the n-type polysilicon layer. A pre-doped and pre-activated p-type polysilicon layer is then applied over the oxide layer and the thin layer of silicon dioxide. The p-type polysilicon layer is then planarized down to the height of the n-type region to form p-type regions, wherein the thin silicon dioxide layer may act as a planarization stop. The p-type and the n-type regions are isolated from one another by the portions of the thin silicon dioxide layer, which become the isolation barriers.
Once the p-type and n-type regions (separated by the isolation barriers) are fabricated, resist masks are formed over each isolation barrier such that open areas are positioned over the n-type regions and the p-type regions. The n-type regions and the p-type regions are then simultaneously etched and the resist masks removed such that the n-type region and the p-type regions are each substantially bifurcated into n-type areas and p-type areas, respectively.
A layer of undoped semiconductive material is disposed over the exposed portions of the oxide layer, the n-type areas, the p-type areas, and the isolation barriers. A layer of insulative material is placed over the undoped semiconductive material. A layer of metal gate material is then placed over the insulative material layer. A layer of barrier oxide is then disposed over the metal gate material layer.
The barrier oxide layer is masked and etched down to each of the n-type areas, the p-type areas, and the isolation barriers to form gates. An isolation material is applied over the gates, the n-type areas, the p-type areas, and the isolation barriers. The isolation material is etched, leaving end caps abutting edges of the gates.
A layer of passivation material is disposed over the gates, the n-type areas, the p-type areas, and the isolation barriers. The passivation layer is then masked and etched to expose a portion of the n-type areas, the p-type areas, and the isolation barriers.
Poly plugs are disposed within the etched areas in the passivation layer to contact the n-type areas, the p-type areas, and the isolation barriers to form a CMOS TFT structure. Thus, the entire CMOS TFT structure is built with only four mask steps with the n-type and p-type areas being formed by two of these mask steps, which eliminates time-consuming, multiple masking steps required by known fabrication techniques. This, in turn, reduces the cost of manufacturing the CMOS TFT.
As discussed above, the present invention utilizes deposited layers of pre-doped and pre-activated n-type and p-type polysilicon layers. However, it is understood that ion implant steps can be used to vary device characteristics.
Furthermore, it is, of course, understood that, although the present invention is described in terms of a CMOS TFT, the techniques can be applied to other MOS structures such as PMOS and NMOS structures.